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Chromosome Mapping: Notes on Chromosome Mapping | Biology

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Here are your notes on chromosome mapping!

It has been established that in Dihybrid genetic crosses a recombination frequency of less than 50 per cent for a pair of genes indicates linkage, i.e., being on the same chromosome, Morgan and Sturtevant found that the recombination frequency varies for different mutant gene pairs.

For example, the mutants black body (b) and vestigeal wings (vg) showed a recombination frequency of 18 per cent while black body (b) and cinnabar eye (cn) showed 9 per cent. Thus, cross-over between b and vg are twice as frequent as those between b and cn.

The difference in the recombination frequencies was found to be associated with the distance between the genes. The genes located farther apart in a chromosome undergo crossing-over more frequently than the genes laying nearby. This shows that the distance between b and vg should be about twice as that between b and cn. Hence, the recombination frequencies can help in the determination of the positions called loci, of the genes in the chromosomes and in preparing chromosome maps.

Thus, a chromosome map is a graphic representation of a linkage group in the form of a line which shows by points the sequence of the genes and the relative distances between the genes it contains. A chromosome map is also called a linkage map or genetic map. The method of preparing chromosome map of a species is known as chromosome mapping.

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Chromosome mapping is based on two under mentioned genetic principles. They are –

1. That the genes are arranged in a linear order in the chromosome.

2. That the frequency of crossing over between two genes is directly proportional to the distance between them in the chromosome. Since the frequency of crossing over is used in preparing chromosome maps, the latter are also called cross over maps.

Chromosome map unit:

A frequency of one per cent crossing over between two genes is known as one unit of map distance between these genes and it is termed a Morgan after the name of T. H. Morgan.

How to prepare chromosome maps:

Certain examples are quoted here to explain the technique of preparing a chromosome map. Suppose the five genes A, B, C, D and E are located in a single chromosome. If the crossing over percentage between A and B is 10 Morgan’s, these genes are plotted 10 arbitrary units apart in a linear scale (Fig. 37.12 A).

If the crossing over percentage between B and C is three, the crossing over percentage between A and C will determine the position of C, since it is three units from B towards A or away from A. It is assumed that A-C cross over rate is 13 per cent, and therefore, C will be plotted 3 Morgan’s on the right side of B (Fig. 37. 12 B).

As per above mentioned examples, the location of the genes D and E can be plotted. Let A- D cross over rate be 5 per cent and B-D cross over rate also 5 per cent. D will then be plotted between A and B, i.e., 5 Morgan’s from each (Fig. 37. 12 C). If cross over rate between C and E happens to be two per cent and between B and E 5 per cent. E will be plotted two Morgan’s on the right side of C (Fig. 37. 12D).

Morgan and Sturtevant got recombination frequencies of (i) 18 between the genes B for black body and for vestigial wings, (ii) 9 between the genes b for black body and cn cinnabar eye and (iii) 9.5 between the genes cn for cinnabar eye and vg for vestigial wings. If the gene b is plotted at one end of the chromosome, the gene vg will be 18 Morgan’s away from it (Fig. 37. 13 (a). The gene cn being 9 Morgan’s from b and 9.5 Morgan’s from vg will be plotted approximately in the middle between the genes b and vg. Thus, the sequence of genes on the chromosome is b, cn and vg.

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The recombination frequency of 9.5 was obtained between cn and vg. This revealed the sequence to be b – cn – vg with a total distance of 9 + 9.5 = 18.5 map units which is close to the observed 18 map units for b — vg. If it were cn – b – vg, c – vg, distance should be 9 + 18 = 27 map units apart. Using this method the relative distances and position of hundreds of mutant genes of Drosophila have been mapped.

All these mutants cluster together to form four groups of linked genes called linkage groups. Drosophila possesses four types of chromosomes, i.e., I, II, III and IV. Each linkage group remains associated with a particular chromosome. No gene has been identified on the Y chromosome.

Chromosome maps were first prepared for Drosophila, maize and Neurospora. Now the linkage maps have also been made for certain other organisms. By a somewhat different technique the chromosome maps of certain viruses and for the circular DNA of some bacteria have also been prepared.